Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $29.56   11.239%   $32.8822   $32.88   $32.90   $32.90  
  Purchase #2 $141.90   12.545%   $159.7014   $159.70   $159.70   $159.70  
  Purchase #3 $111.25   6.262%   $118.2165   $118.22   $118.20   $118.20  
  Purchase #4 $19.35   8.834%   $21.0594   $21.06   $21.05   $21.10  
  Purchase #5 $242.43   7.292%   $260.108   $260.11   $260.10   $260.10  
  Purchase #6 $214.72   11.154%   $238.6699   $238.67   $238.65   $238.70  
  Purchase #7 $187.40   6.706%   $199.967   $199.97   $199.95   $200.00  
  7 purchase totals = $946.61       $1,030.6044   $1,030.61   $1,030.55   $1,030.70